Swift Array extension to create interleaved arrays and permutations.

Permutations.swift

extension Array {
    var decompose: (head: Element, tail: [Element])? {
        return count > 0 ? (self[0], Array(self[1..<count])) : nil
    }
}

extension Array {
    func interleave(x: Element) -> [[Element]] {
        guard let (head, tail) = decompose else { return [[x]] }
        let start = [x] + self
        return [start] + tail.interleave(x).map { [head] + $0 }
    }

    var permutations: [[Element]] {
        guard let (head, tail) = decompose else { return [[]] }
        return tail.permutations.flatMap { $0.interleave(head) }
    }
}